The application or spraying of coatings onto various substrates by the use of spray guns is well known. Such spraying typically has been accomplished by means of several types of atomizers, including the following: (1) conventional high pressure air blast atomizers; (2) HVLP (high volume, low pressure) air blast atomizers; (3) airless, high pressure atomizes; and (4) air assisted airless atomizers. Each of these types of atomizers have their own characteristics which make them more or less desirable to be used in the application of a particular coating material or paint, or on a particular substrate, or in a particular setting or location. Other factors, such as govermental regulations, now are beginning to dictate the type of atomizer which can be used.
Recent actions by governmental agencies include efforts to reduce the atmospheric emissions of hydrocarbon solvents that form ozone in the lower atmosphere. Two regulatory approaches have especially affected the spray coatings industry. One approach is the lowering of the permitted levels of hydrocarbon solvent or volatile organic content (VOC) in paints. This approach would reduce emissions because, if the same amount of solids is applied, less solvent would be sprayed and released to the atmosphere. Another approach is the limiting of the allowable air pressure that can be employed for atomizing paint in air blast type spray guns. Spraying with lower atomizing air pressure increases the spray droplet size, which reduces the number of very small droplets that can be carried away with air currents (overspray). This results in a higher transfer efficiency, or, in other words, more of the paint solids end up on the surface being painted and less is wasted.
Lower VOC regulatory limits are resulting in the development of paints with less hydrocarbon solvents (higher solids) or with water as the solvent (water based). The high solids paints are higher in viscosity and harder to atomize, and the water based paints dry or cure more slowly, especially in cool and humid weather conditions. These new kinds of paints have suggested the need for new or different applicators or spraying methods.
The above mentioned limit on atomizing air pressure has resulted in a multitude of new spray guns being offered for sale. They are referred to as HVLP (high volume, low pressure) spray guns. During use, the air pressure utilized in such a gun is less than or equal to 10 psig, as measured just inside the air exit orifice. The combination of lower VOC paints and lower atomizing air pressure has resulted in a decrease in atomizing quality (average droplet size in the spray). This makes it difficult for the operator of the spray gun to achieve the desired finish appearance that satisfies the customer.
Initially, most HVLP systems included a turbine to supply a high volume of air at low pressure to a spray gun. In the process of compressing air, the air is naturally warmed, so that turbine spraying systems can provide warm atomizing air.
High volume, low pressure (HVLP) spraying is alternately called turbine spraying, since the normal method of supplying air for atomizing paint is a turbine. However, another method of supplying warm air involves a compressed air conversion unit. This is simply a common pressure regulator that reduces the compressed air pressure down to below 10 psig. This low pressure air is then heated to 100.degree. to 200.degree. F. by means of an electric resistance heater.
Both of the above-mentioned methods of providing warm air, turbine spraying and the use of compressed air conversion units, have certain drawbacks. For instance, turbines have moving parts that can break down and that require occasional maintenance. Turbines also require electrical power to operate, which must be explosion proof when used in areas that are classified as electrically hazardous according to the National Electric Code. Also, the control of air temperature is not precise with turbines. In most cases, the temperature is only controlled by the length of air hose connected between the turbine and the spray gun. Further, the initial capital costs of a turbine system is relatively high. This expense is especially high if pressures higher than about 5 psig are desired, since turbines employed in refinish shops are usually so limited. Likewise, a compressed air conversion unit suffers from similar drawbacks. The temperature control of these units is typically accomplished with a thermostat employing on-off control and resulting in significant temperature cycling. In order to make this type of unit suitable for electrically hazardous areas, expensive purging or an expensive explosion proof mounting box would be required.
There exists a need for a source of warm atomizing air for spray guns which is inexpensive, requires little or no maintenance, has simple and precise temperature adjustment, and is explosion proof.